The high frequency amplifier usually has such characteristics that the efficiency increases as the output level becomes close to a saturation level. On the contrary, there is a problem in that the efficiency is low when the output level is low. For instance, if the high frequency amplifier is used in a system having a wide dynamic range of the output power, the efficiency in a low output power becomes low. In this case, it is a task to enhance the efficiency in a low output power.
A conventional high frequency amplifier is devised to enhance the efficiency in a low output power like a high frequency amplifier disclosed in Non Patent Document 1, for example, by connecting amplifiers having different sizes in parallel to each other and by switching the amplifiers in accordance with the output level so that an amplifier having a larger size is activated if the output level is high while the other amplifier having a smaller size is activated if the output level is low.
In addition, Patent Document 1 discloses a method of switching a size of the amplifier by using a switch made up of transistors.
In addition, Patent Document 2 discloses an output matching circuit in which an output impedance of an amplifier is matched to be a characteristic impedance of 50 ohms (Ω) in any case when the amplifier is switched.
In addition, Patent Document 3 discloses a devised structure for enhancing the efficiency in a low output power by controlling a collector voltage of an amplifier. It also discloses a devised structure for enhancing the efficiency in a low output power by changing an output matching circuit with a switch simultaneously when a size of the amplifier is changed.
In addition, Patent Document 4 discloses a devised structure for enhancing the efficiency in a low output power by switching an output matching circuit with a switch when a size of the amplifier is changed.
Further, Patent Document 5 discloses a devised structure of an amplifier in which sizes of two stages of amplifiers are switched in accordance with an output level. A switch is provided between the stages of amplifiers, and a switch provided to the amplifier to be turned off is switched off so that isolation is enhanced for suppressing an oscillation.
Patent Document 1: JP 2000-278109 A
Patent Document 2: JP 2003-046340 A
Patent Document 3: JP 2002-353751 A
Patent Document 4: JP 2004-134823 A
Patent Document 5: JP 2003-087059 A
Non Patent Document 1: J. H. Kim, etc., “A Power Efficient W-CDMA Smart Power Amplifier With Emitter Area Adjusted For Output Power Levels”, 2004 IEEE International Microwave Symposium (MTT-S) Digest, pp. 1165-1168.